Leonard plays half-back and made his provincial debut for Waikato in the 2005 National Provincial Championship. He was in the Chiefs wider training group for the 2006 Super 14. After playing for Waikato in their championship winning 2006 Air New Zealand Cup side, Leonard was selected for the Chiefs in the 2007 Super 14. Leonard had a successful Super 14 where he played for Byron Kelleher who missed the first seven rounds of the competition to participate in the All Blacks reconditioning programme. Following the 2007 Super 14 he was selected for the All Blacks squad for the 2007 mid-year Tests and 2007 Tri-Nations. He was one of only two uncapped players named in the squad, and All Blacks coach Graham Henry said of Leonard's selection "Brendon [Leonard] has been the best attacking scrum-half in the Super 14 this season and has forced his way into the side based on form." He made his Test debut against France at Eden Park on 2 June 2007. He came on as a substitute for Piri Weepu in the 64th minute of the All Blacks 42-11 victory. He then scored his first Test try against South Africa on 14 July 2007. On 12 August 2013, Leonard left New Zealand as he signed for new Italy franchise Zebre in the Pro 12 from the 2013-14 season. On 22 January it was announced Leonard had signed for the Ospreys, mainly to provide experienced cover for Welsh international Rhys Webb during the 2015 Rugby World Cup.